two if they are placed among weed of a different colour--from green to brown, or the like. Within certain limits still more rapid changes of colour take place. If kept in the dark, or if placed on a white background (for example, in a porcelain dish) in the light, the Prawn quickly becomes nearly colourless, by contraction of the chromatophores, a transparent bluish tint alone remaining, due to a substance which diffuses from the chromatophores into the fluids of the body. In natural conditions this phase is assumed at night; and the interesting observation has been made that Prawns kept in the dark continue for three or four days to show a periodic expansion and contraction of the chromatophores, corresponding to the alternation of day and night. It seems that the rhythm of light and darkness has become impressed on the chromatophore system of the animal, and the movement of the pigments is regulated by something analogous to memory. [Illustration: FIG. 41--THE COMMON PORCELAIN CRAB (_Porcellana longicornis_), SLIGHTLY ENLARGED, AND ONE OF THE THIRD MAXILLIPEDS DETACHED AND FURTHER ENLARGED TO SHOW THE FRINGE OF LONG HAIRS] It has already been mentioned, in dealing with the Lobster, that certain Crustacea have the power of voluntarily throwing off some of their limbs (autotomy). In many cases, as in the Lobster, this power is mainly of use in enabling the animal to discard an injured limb; but there are some Crustacea which seem to adopt it as a means of escaping from the attack of an enemy. On our own coasts the shore collector will often find, on turning over a large stone, one or more specimens of the little Porcelain Crabs (_Porcellana platycheles_, or _P. longicornis_--Fig. 41) clinging to its under-side. If these Crabs be seized by one of the large claws, they frequently leave the claw in the captor's hand and scuttle off without it; and it cannot be doubted that, as in the case of lizards and other animals which have a similar power of self-mutilation, this habit often enables them to escape from their natural enemies.